Power to speed ratio for displacement boats

Near as I can tell these are the main factors of boat design, not counting wallet thickness, from a power point of view. The important thing to me here is that I get the order right. This is from most important factor to least:

(1.) Displacement: All other things being equal it takes less energy to move less mass.
(2.) Slenderness: When displacement is equal, the slender boat takes the least energy to accelerate.
(3.) Entry: When the slenderness is equal, fineness of entry comes into play.
(4.) Wetted area: etc.
(5.) Sectional profile: etc.
(6.) Skin surface quality: etc.

If I'm looking at this right, then at the same displacement, a box end barge made with a l/b ratio of 10:1 will take less power to push up a certain speed than round sectioned catboat with a l/b of 2:1. Does that seem right?

.. The important thing to me here is that I get the order right.....Is there something I am missing?

Click to expand...

Yes, the SOR.

Without knowing the SOR, all you're doing is taking one element, one chain or link, if you like, and assume this one chain is the absolute, in terms of the design. It is not.
There are many ways to get the best out of the hull, hydrodynamically speaking, but it is all indicated by the SOR. Since the SOR may imply or infer a hull shape that is very draggy and not particularly useful at all. Expect...it satisfies the SOR.

In layman's terms, it is like thinking about the size and shape and quality of an egg, when making a Victoria sponge cake, yet ignoring all the other ingredients. Since you're assuming that THE egg is all that matters. There is more to a Victoria sponge cake, than just the eggs!

... I'm still trying to understand things in the most general, first principles sort of way first.

Click to expand...

Then I would suggest you start reading some very good books on the basics of hydrodynamics. That will make things clearer and more complex at the same time as the more your learn leads to yet more questions!!

Read "The Nature of Boats" by Dave Gerr. In developing a refined SOR, you don't need to have any clue about the beam/length ratios or anything else in regard to hydrodynamics. The SOR typically forces your (the designer's) hand in this regard. You may desire one aspect, but other portions of the SOR dictate you (the designer) take a somewhat different approuch. It's this very thing that has nearly every designer, NA or engineer mostly bald or if like me, still wallowing in baldness denial. The only time this doesn't apply is when developing a very narrow set of variables, like that of a racer. On these, the compromises are still quite confounding though in general, you stay within the rules and class requirements, cheat where you can get away with it (if you don't you'll be mid pack) and cut it as close as you dare, just to remain competitive. I've seen sailboat racers that were cut so thin to the line, the first time they really torqued up the back stay, it literally folded the yacht in half. This tends to makes SOR decisions easier, though in this case maybe a tad too close to the line.

I confess, that from what I know about hydrodynamics, aerodynamics seems easier. With aero if you know the shape and the reynolds number, you can very easily ballpark everything you need to know. With boats that doesn't work. Graph some random coefficient between different boats and instead of nice flat lines, or smooth exponential curves, you get lines that wave all over the damn place with weird low and high points at certain fractions of froude numbers...which makes sense from a wave making resistance point of view...but it makes it really hard to guess a ball park figure.

The reason I don't want to make an SOR yet is because I have don't want to be the kind of "customer" that I have learned to hate over the years. I supplement my income with a little construction contracting. Light, strong, cheap material, fast labor. I can give you any combination of those three that you want, but I can't give you all four...but that's what people ask for. I don't want to be the guy asking for a hurricane proof, 40' ULDB papercrete boat for $5k.

.. With boats that doesn't work. Graph some random coefficient between different boats and instead of nice flat lines, or smooth exponential curves, you get lines that wave all over the damn place with weird low and high points at certain fractions of froude numbers...which makes sense from a wave making resistance point of view...but it makes it really hard to guess a ball park figure....

Click to expand...

Which is the reason why you need to understand the SOR rather than focusing on the absolutes!!!

Adding to JSL's comment, the phenomenon of "Free Surface Flow", where the atmosphere meets the water, complicates things by an order of magnitude or more. Designing a submarine hull is relatively easy when submerged, compared to surface craft.

Aero and hydrodynamics are quite similar, using many of the same formulas, though water being about 50 times more viscous. If you can handle basic 12th grade math, neither of these subjects are very difficult, though truly understanding them, will take considerably longer, for many of us a life time.

Adding to JSL's comment, the phenomenon of "Free Surface Flow", where the atmosphere meets the water, complicates things by an order of magnitude or more. Designing a submarine hull is relatively easy when submerged, compared to surface craft.

Click to expand...

Yeah, that seems to be the case to me. Just rotate a NACA 0015 and you have a pretty OK sub hull, mostly regardless of displacement or desired speed. Whereas displacement designs don't scale well because of waves, and the fact that resistance to speed depends on froude number, which changes at the boat heels. D

Adding to JSL's comment, the phenomenon of "Free Surface Flow", where the atmosphere meets the water, complicates things by an order of magnitude or more. Designing a submarine hull is relatively easy when submerged, compared to surface craft.

Click to expand...

Fred is correct. Free surface effects are a major complication for surface vessel hydrodynamics.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.